This invention relates to an exposure apparatus and a device manufacturing method. More particularly, the invention is concerned with an exposure apparatus for use in an exposure process for the manufacture of devices such as a semiconductor devices (e.g., ICs or LSIs), image pickup devices (e.g., CCDs), display devices (e.g., liquid crystal panels) or sensors (e.g., magnetic heads), for example. In another aspect, the invention is concerned with a device manufacturing method for manufacturing devices such as described above.
An optical element such as a transparent plate, a lens or a prism used in a projection optical system or an illumination optical system of a projection exposure apparatus is formed on its bottom surface with an anti-reflection film (optical thin film). The provision of such an anti-reflection film is to efficiently direct light, from a light source, to a photosensitive substrate and also to prevent a flare or a ghost from impinging on the photosensitive substrate. Since a light source of a projection optical system produces strong ultraviolet rays, intensive ultraviolet light is projected on the surface of an optical element of an illumination optical system or of a projection optical system. Particularly, in a case wherein the light source comprises an excimer laser which emits pulse light in the ultraviolet region, the energy of ultraviolet light per unit time is very large. As a result, the spectral reflectivity characteristic of an anti-reflection film or the absorptivity at various surfaces may slightly change, to cause a change in spectral transmissivity. Generally, an illumination optical system and a projection optical system include optical elements having surfaces of a number of a few tens in total. Thus, even if the spectral transmissivity change per one surface is small, it may cause a large spectral transmissivity change in total.
Spectral reflectivity of an anti-reflection film changed by irradiation of ultraviolet rays may change, if irradiation of ultraviolet light is stopped, to be restored to its original spectral reflectivity characteristic. Thus, the transmissivity of the illumination optical system or projection optical system changes with the state of operation of the exposure apparatus. Such a phenomenon may attribute to water content or organic materials within the film from being disengaged by intensive ultraviolet rays entered into the film while, on the other hand, in a state in which no ultraviolet light is projected, the water content or organic materials within the environment being absorbed by the film.
Generally, the amount of exposure to be supplied to a photosensitive substrate may be controlled by receiving a portion of ultraviolet light by the use of a photodetector disposed in the illumination optical system and by detecting the light quantity upon the photosensitive substrate on the basis of the received light quantity and of the ratio between the received light quantity and the exposure amount, which may be determined beforehand. Thus, if the transmissivity of an (illumination) optical system after such a photodetector and of the projection optical system varies due to the phenomenon described above, the ratio between the light quantity as projected on the photodetector and the light quantity supplied onto the photosensitive substrate may change, causing an error in the detected value of the exposure amount. As a result, the photosensitive substrate cannot be exposed with a correct exposure amount.
Additionally, there may be a case wherein a change in spectral reflectivity (spectral transmissivity) of an anti-reflection film causes a change in an illuminance distribution upon a photosensitive substrate.
It is an object of the present invention to provide an exposure apparatus and a device manufacturing method, by which a substrate can be exposed with a correct exposure amount.
In accordance with a first aspect of the present invention, there is provided an exposure apparatus for illuminating a mask with an illumination optical system to expose a substrate in accordance with a pattern of the mask, said apparatus comprising: transmissivity maintaining means for maintaining, substantially constant, a transmissivity of a portion of or the whole of said illumination optical system.
In accordance with a second aspect of the present invention, there is provided an exposure apparatus for illuminating a mask with an illumination optical system and for projecting a pattern of the mask onto a substrate through a projection optical system, said apparatus comprising: transmissivity maintaining means for maintaining, substantially constant, a transmissivity of a portion of or the whole of a system provided by said illumination optical system and said projection optical system.
In accordance with a third aspect of the present invention, there is provided an exposure apparatus for illuminating a mask with light from a light source and through an illumination optical system, and for projecting a pattern of the mask onto a substrate through a projection optical system, said apparatus comprising: transmissivity maintaining means for projecting light, from the light source, to said illumination optical system and said projection optical system so as to maintain, substantially constant, a transmissivity of said illumination optical system and said projection optical system. The light projection by said transmissivity maintaining means may be performed, basically, before practically exposing the substrate.
In the first to third aspects of the present invention, said illumination optical system may include optical elements having anti-reflection films formed on their light entrance and exit surfaces.
The illumination optical system may include a reflection mirror having an intensified reflection film.
The illumination optical system and the projection optical system may include optical elements having anti-reflection films formed on their light entrance and exit surfaces, wherein said optical elements may include lens elements.
In the first to third aspects of the present invention, said illumination optical system may include light dividing means for dividing light from a light source, and said apparatus may further comprise photoelectric converting means for receiving a portion of light from said light source as provided by said light dividing means, and exposure amount control means for detecting and controlling the amount of exposure of the substrate on the basis of an output of said photoelectric converting means.
The transmissivity maintaining means may maintain, substantially constant, the transmissivity of an optical system between said light dividing means and the substrate.
The apparatus may further comprise transmissivity measuring means for measuring transmissivity of an optical system between said light dividing means and the substrate.
The transmissivity measuring means may include said photoelectric converting means as well as second photoelectric converting means at least having a light receiving portion provided on substrate holding means, for holding the substrate and being movable, wherein said photoelectric converting means and said second photoelectric converting means may operate to perform photoelectric conversion of lights impinging on them while said second photoelectric converting means may be disposed opposed to a light exit surface of an optical system between said light dividing means and the substrate, wherein a ratio of outputs of said photoelectric converting means and said second photoelectric converting means may be calculated, and wherein the transmissivity may be determined on the basis of the calculated output ratio.
In the first to third aspects of the present invention, said transmissivity maintaining means may maintain, substantively constant, the transmissivity of an optical system between said light dividing means and the substrate, by projecting light from the light source to the optical system between said light dividing means and the substrate.
While predicting the transmissivity of an optical system between said light dividing means and the substrate on the basis of an output of said photoelectric converting means, light from the light source may be projected to the optical system between said light dividing means and the. substrate, separately from a practical exposure operation, to thereby set the transmissivity between said light dividing means and the substrate at a desired value.
The amount of change of transmissivity of an optical system between said light dividing means and the substrate may be predicted on the basis of an output of said photoelectric converting means and of time information, wherein, when the amount of change of transmissivity exceeds a predetermined value, a predetermined quantity of light from the light source may be projected to the optical system between said light dividing means and the substrate, separately from a practical exposure operation, to thereby set the transmissivity of the optical system between said light dividing means and the substrate to a desired value.
A predetermined quantity of light from the light source may be projected, with a certain periodicity, to an optical system between said light dividing means and the substrate, to thereby maintain, substantially constant, the transmissivity of the optical system between said light dividing means and the substrate, wherein said predetermined periodicity may be xe2x80x9conce at a predetermined time per one day (twenty-four hours)xe2x80x9d or xe2x80x9conce at a predetermined time per two days (forty-eight hours)xe2x80x9d.
The transmissivity measuring means may measure the transmissivity of an optical system between said light dividing means and the substrate, wherein, when the measured value is out of a predetermined range, a predetermined quantity of light from the light source may be projected to the optical system between said light dividing means and the substrate, separately from a practical exposure operation, to thereby set the transmissivity of the optical system between said light dividing means and the substrate to a desired value.
In these cases, usually, before a practical (initial) exposure operation, a predetermined quantity of light from the light source may be projected to an optical system between said light dividing means and the substrate to thereby set the transmissivity of the optical system between said light dividing means and the substrate to a desired value.
The optical system between said light dividing means and the substrate may include a projection optical system for projecting a pattern of the mask onto the substrate, wherein said apparatus may further comprise correcting means for projecting light from the light source to an optical system between said light dividing means and the substrate, to thereby compensate for a change in optical characteristic produced in said projection optical system.
The correcting means may include predicting means for predicting an amount of change of the optical characteristic at a predetermined time, and adjusting means for adjusting said apparatus in accordance with the predicted amount of change.
The optical characteristic may include a projection magnification of said projection optical system, wherein said adjusting means may include at least one of (i) moving means for moving one of a lens element of said projection optical system and the mask in an optical axis direction of said projection optical system, (ii) pressure changing means for changing a pressure of air between lenses of said projection optical system, and (iii) wavelength changing means for changing a wavelength of light from the light source.
The optical characteristic may include an imaging position of the mask pattern through said projection optical system, wherein said adjusting means may include at least one of (i) moving means for moving the substrate in an optical axis direction of said projection optical system, (ii) pressure changing means for changing a pressure of air between lenses of said projection optical system, and (iii) wavelength changing means for changing a wavelength of light from the light source.
In the first to third aspects of the present invention, said light apparatus may include an excimer laser as a light source for the exposure operation, wherein said excimer laser may comprise one of a KrF excimer laser and an ArF excimer laser.
The illumination optical system may serve to define a slit-like illumination region of a width smaller than the width of the whole pattern of the mask to be transferred, and wherein the mask and the substrate may be scanned relative to said illumination optical system in a direction perpendicular to the lengthwise direction of the slit-like illumination region by which the whole mask pattern may be transferred to the substrate.
The illumination optical system may serve to define an illumination region of the same size as the whole pattern of the mask to the be transferred.
In accordance with a further aspect of the present invention, there is provided a device manufacturing method which includes a process of transferring a device pattern onto a substrate by use of an exposure apparatus such as described above.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.